It is well known in the art to improve the properties of light metals such as Al, Mg, etc. (i.e., the matrix metal) by dispersing a variety of filler particles (e.g., ceramics) throughout the metal. Common filler particles include carbon/graphite, alumina, glass, mica, silicon carbide, silicon nitride, wollastonite, potassium titanate fiber, aluminosilicate (e.g., Kaowool), zirconia, yttria, inter alia. Such enhanced metals are often referred to as "metal matrix composites" or MMCs. The filler particles (a.k.a. reinforcements) may be essentially equiaxed, or elongated (e.g., whiskers and fibers), and serve to improve one or more of the mechanical properties (e.g., strength, toughness, lubricity, friction, fatigue resistance, wear resistance, etc.) of the composite over the properties of the matrix metal alone. Popular elongated particles (hereafter, fibrils) typically have an aspect ratio (i.e., length divided by diameter) of between, about 3 to about 20, and may be as high as about 50. The lengths of the fibrils vary from about 50 to about 500 microns, and their diameters are generally less than about 10 microns. Typically, the reinforcing particles will constitute about 3% by volume to about 30% by volume of the MMC if the particles are fibrils, but may constitute as much as 70% by volume when the fillers are small equiaxed particles.
It has been heretofore proposed to make MMCs by either one of two processes. In one process, the filler particles are simply mixed with the metal while molten, and the mixture cast into an appropriate mold for shaping the finished product. In the second process, a self-supporting, net shape (i.e., size and shape of the finished product or portion thereof) porous preform of the filler particles is first formed and then subsequently impregnated with the matrix metal by well known wicking or pressure filling techniques.
Heretofore, preforms have been made by vacuum casting, where a 5 volume percent whisker/water slurry is drawn through a screen leaving behind a mat of whiskers which is further densified to 15-25 volume percent by pressing. This process has a number of disadvantages. First, vacuum casting is limited to shapes with two-dimensional complexity, and dimensional control is poor (at best .+-.0.10 cm/cm). More complex shapes must be machined from vacuum cast blocks, but this adds cost to the process. Second, the whiskers in vacuum cast preforms are oriented in a random planar fashion, giving rise to planes of weakness in the preform. Third, inorganic binders such as colloidal silica must often be added to give the preform sufficient strength to withstand handling. These binders may become entrained in the MMC during infiltration and can have a detrimental effect on MMC properties if they cluster together.
Preforms have also been made by injecting a mixture of the filler particles and an organic binder into a suitable mold, removing the binder and then, optionally, bonding and the particles together into a self-supporting structure. One known such technique for making preforms comprises mixing the filler particles uniformly throughout a fugitive binder (e.g., wax, polystyrene, polyethylene, methyl cellulose/H.sub.2 O gel, etc.), injecting the binder-particle mixture into a mold, and removing (e.g., burning out, volatizing or dissolving) the binder. In some cases (e.g., with certain materials, or with low particle loadings), it may be desirable to bond the particles together following binder removal and before impregnating them with metal. Particle bonding, if used, may be achieved (1) by sintering, (2) by initially providing the particles with a coating of colloidal silica or alumina which, upon heating, acts like a high temperature inter-particle glue, or (3) by oxidizing the particles to hold them together. When SiC is used as the reinforcement, the SiC particles can be bonded together by heating the particles to above 600.degree. C. in air to form SiO.sub.2 in situ on the surfaces which they bond the particles each to others.
Another more recently developed technique for making preforms involves mixing the filler particles with certain prepolymers used to produce a fugitive open-cell foam such that the particles migrate to, and align themselves with, the ligaments formed in the resulting foam. This technique is described in more detail in copending U.S. patent application Powell et al, Ser. No. 08/169,251 filed Dec. 20, 1993 and assigned to the assignee of the present invention.
After, the preform is made it is transferred to a metal-filling station where it is impregnated with the desired matrix metal (e.g., aluminum). Metal impregnation may be accomplished by evacuating air from the porous preform, contacting it with molten metal, and allowing the metal to settle or wick into the preform. In one such technique, the preform is laid atop a solid mass of the matrix metal, and together therewith, heated in flowing nitrogen to above the melting point of the metal until the metal wets the particles and wicks into the preform. Preferably, however, the metal will be forced into the preform under pressure (e.g., as by squeeze casting).
Preforms made heretofore tended to distort and lose their shape when heated to remove the binder. Moreover, a problem with preforms made by injection molding, is the time required for, cost of, and environmental considerations associated with, burning off of the large amounts of organic binder used therewith. Still further, preforms made heretofore tend to lack durability in that they are quite delicate and fragile, and accordingly can easily crack during handling and/or filling with metal. Regardless of these difficulties, the use of preforms is still considered by many to be the preferred way to make MMCs owing to the ability to incorporate higher whisker volume fractions than is possible by the direct casting of whiskers dispersed in the molten metal, and the ability to reinforce select areas of a casting without having to reinforce the entire casting.
Copending U.S. patent application Sokol et al. (Attorney Docket Ser. No. G-7971), filed concurrently herewith and assigned to the assignee of the present invention, discloses an improved, durable, filler preform for making heterogeneous MMCs which have good wear-resistance properties.
The MMCs produced by Sokol et al. have two distinct interpenetrating metal-containing regions. One "particle-rich" region comprises about 60% to about 99% by volume of the MMC, and contains a multiplicity (i.e., ca. 5% to ca. 70% by volume) of discrete filler particles dispersed throughout the metal. The particles preferably comprise fibrils intertwined one with the next for enhanced preform strength. The second of the interpenetrating regions comprises about 1% to about 40% by volume of the MMC and is devoid of any filler particles, i.e., is "particle-free". The second, or particle-free, region pervades the composite in the form of a three-dimensional, open-cell reticulum of randomly oriented ligaments interconnecting a plurality of nodes and defining a plurality of interconnected interstitial cells which vary in size from about 50 microns to about 10,000 microns. The first, or particle-rich, region fills the interstitial cells defined by the second, or particle-free, region. Overall, the MMC will comprise about 2% to about 70% by volume of the particles. When fibrilous particles are used, loadings of ca. 30%-40% by volume, maximum, are used. The fibrils will have lengths varying between about 1 micron and about 500 microns, have diameters less than about 10 microns, and have aspect ratios (i.e., length/diameter) varying between about 3 and about 50 depending on the composition of the particular filler particle being used. Filler particles particularly useful with the present invention include carbon/graphite, alumina, glass, mica, silicon carbide, silicon nitride, wollastonite, potassium titanate fiber, aluminosilicate (e.g., Kaowool), zirconia, and yttria.
Sokol et al.'s preform comprises a porous, heterogeneous mass of discrete filler particles comprising about 2% to about 70% by volume of the preform. The particles may or may not be bonded to each other depending on the particular fillers being used and the amount thereof. In this regard, if the preform is sufficiently durable and self-supporting without separate interparticle bonding, no such bonding is needed. The particle mass is pervaded with a three-dimensionally, reticulated, particle-free network. In a preferred embodiment, the particle-free network initially comprises a plurality of randomly oriented, fugitive polymeric ligaments interconnecting a plurality of nodes dispersed throughout the particle mass (i.e., a polymeric foam). Prior to filling the preform with metal, the polymer is volatized or burned-off leaving a network of capillaries in its stead conforming to the shape of the original polymeric foam. In another embodiment, the particle-free reticulated network comprises a plurality of randomly oriented metal ligaments interconnecting a plurality of nodes dispersed throughout the particle mass (i.e., a metal foam). The metal foam is not removed and remains with the preform as well as the MMC made therefrom.
It is an object of the present invention to provide a unique process for making heterogeneous MMC's made from preforms of the type described in Sokol et al. (G-7971).